Answer: perhaps it’s better to talk about prices than about value, which is quite undetermined, for the time being. It’s value may be considered though as mainly representative in case you use zircon for jewelry, and as relative (diamond is better as harder) for industrial use.
There are nowadays three main zircon producing regions: Brazil, the Tschech Republic and Pakistan. Of those, the Brazil zircon, mainly sold in Europe as CEN zircon, gets prices that may go beyond the 1000 USD a karat in selling markets (shop). The CZ zircon, from the Tschech Republic, better in quality than all the others as being harder (reaches 2,18 hardness, besides the 1,95 of CEN and 2,34 of diamond = approx values), is lately bargain sold at 1 USD a karat in providing markets and may reach approximative sales values vaying from almost nothing to around 200 USD a little bit all over. The Pakistan zircon seems to have restrained its activity around the islamic world and reaches approximative selling prices (shop) of around 100 USD a karat in Istanbul.